As well known in the art, in an internal combustion engine mounted on a vehicle, so-called idle rotation speed control is performed, in which an engine rotation speed is converged to a target idle rotation speed by increasing or decreasing intake air quantity (intake air amount) using a throttle valve or the like in accordance with a deviation between the target idle rotation speed and an actual rotation speed of the internal combustion engine detected by the crank angle sensor or the like.
On the other hand, there have been proposed variable compression ratio mechanisms capable of changing a mechanical compression ratio of the internal combustion engine (expansion ratio) by varying a piston stroke characteristic in accordance with an engine operating condition, by the present applicant and others. As an example of idle rotation speed control using such a variable compression ratio mechanism, Patent Literature 1 recites a technology in which an actual compression ratio is detected by a compression ratio sensor and an intake air quantity is corrected in accordance with the actual compression ratio so as to suppress deterioration of convergence of the idle rotation speed control due to a response delay caused upon changing the compression ratio.
Further, Patent Document 2 recites a technology in which idle rotation speed control is performed by conducting ignition timing control in combination with intake quantity control by the throttle valve. During idling, an ignition timing is corrected on the basis of an actual intake quantity detected by an air flow meter, while an intake quantity is increased or decreased by adjusting an throttle opening degree so as to maintain a target idle rotation speed.